Rename ConcurrentUpdate to ConcurrentTransition

ConcurrentUpdate is too generic - one might get a feeling that
it catches all concurrent updates (like optimistic lock does),
when in fact it only catches concurrent state transitions.
Rename FSMLockMixin to ConcurrentTransitionMixin
Update changelog in readme
Merge pull request #53 from knaperek/master

Rename ConcurrentUpdate to ConcurrentTransition